Pros
The decentralized nature of the Institute can give you a home in a little team with relatively little interference from central leadership/the board. Policy programs have a lot of leeway to set their agenda which means that if you have supportive team leadership. My team has amazing people and we get to do some pretty cool stuff sometimes. I also think our pay is pretty good for nonprofit.
Cons
Classic nonprofit work/life balance issues. It sometimes feels hard to do really impactful work as the expectation is to just host a roundtable or write a whitepaper no one will read. Projects aren't resourced to do more than that and (in my team) there's an under-emphasis on connecting work to the outside world. Oh also some benefits don't kick in until after a year of employment which is just sucky and cruel.